pictures of Kunenbo, original mandarin fruit
Along with Kishu, it was the other parent of the now very popular Satsuma mandarin.
It was once a popular variety of mandarin in Japan before it later became eclipsed by the Satsuma variety beginning in the 1600s, which had looser easier to peel skin and fewer seeds.
Kunenbo is a fairly large sized mandarin variety, and is actually more of a tangor-type citrus (mandarin with a small amount of pomelo going back in its ancestry). It is said to have a "noble scent" (probably high level of ocimene in the fragrance) and very flavorful.
There are a few different cultivars of Kunenbo, the one shown in these pictures is probably "Kunenbo A", the good tasting variety.
The Satsuma mandarin probably gets its cold tolerance genes from Kunenbo because Kishu mandarin is not a particularly hardy variety.
Kunenbo ( 九年母 ) translates as "nine year mother" or "ninth century mother", perhaps because it came to Japan from China during the Ninth Century.
Kunenbo is now very rare in Japan, but can still be found in some historical botanical gardens.